This invention relates to a SONET (Synchronous Optical Network) transmission signal processing method and a SONET transmission signal processing apparatus.
A SONET transmission signal has a data transmission rate of 51.840 Mb/sec. Referring to FIG. 1, one frame of the SONET transmission signal is formed from data of 90 columns.times.9 rows in units of 1 byte (8 bits), that is, data totaling 810 bytes (6,480 bits). Accordingly, the SONET transmission signal has a frame length of 125 .mu.s.
One frame of a SONET transmission signal is divided into transport overhead A and synchronization payload envelope capacity B for transmission of the information payload. Transport overhead A is formed from the first three columns and has 9 bytes.times.3 columns=27 bytes allotted to it. Meanwhile, synchronization payload envelope capacity B is formed from the remaining 87 columns and has 9 bytes.times.87 columns=783 bytes allotted to it. Referring to FIG. 2, the first column of synchronization payload envelope capacity B is used as a pass overhead (POH), and the remaining 86 columns of synchronization payload envelope capacity B are used as payload capacity (the portion which substantially transmits the payload).
Data of the following four virtual tributary sizes (hereinafter referred to as "VT sizes") are accommodated in synchronization envelope capacity B:
(1) data of the VT 1.5 size (data transmission rate=1.728 Mb/s),
(2) data of the VT 2 size (data transmission rate=2.304 Mb/s),
(3) data of the VT 3 size (data transmission rate=3.456 Mb/s), and
(4) data of the VT 6 size (data transmission rate=6.912 Mb/s).
In order for data to be accommodated efficiently in synchronization payload envelope capacity B, synchronization payload envelope capacity B is divided into three blocks each including 29 columns as shown in FIG. 3, and fixed stuff formed from one column is inserted into the 30th and 59th columns. Further, data is accommodated into the 84 columns of the blocks (except for the POH and the two units of fixed stuff) in the following manner in accordance with the VT sizes.
(1) When the VT size is 1.5, four columns are allotted as one VT group, and the data is accommodated in synchronization payload envelope capacity B in units of 7 VT groups (4 columns.times.7=28 columns). For example, if the VT sizes of all data accommodated in synchronization payload envelope capacity B are equal and 1.5, as shown in FIG. 4, the first unit is accommodated in the second to the 29th columns (i.e., between the POH and the first unit of fixed stuff); the second unit is accommodated in the 31st to 58th columns (i.e., between the first and second units of stuff); and the third unit is accommodated in the 60th to 87th columns (i.e., in the columns following the second unit of fixed stuff).
(2) When the VT size is 2, three columns are allotted as one VT group, and the data is accommodated in synchronization payload envelope capacity B in units of 7 VT groups (3 columns.times.7=21 columns). For example, if the VT sizes of all data accommodated in synchronization payload envelope capacity B are equal and 2, as shown in FIG. 5, the first unit (21 columns) is accommodated in the second to the 22nd column, the first 7 columns of the second unit are accommodated in the 23rd to 29th columns and the remaining 14 columns of the second unit are accommodated in the 31st to 44th columns, the first 14 columns of the third unit are accommodated in the 45th to 58th columns and the remaining 7 columns of the third unit are accommodated in the 60th to 66th columns, and the fourth unit is accommodated in the 67th to 87th columns.
(3) When the VT size is 3, two columns are allotted as one VT group and the data is accommodated in synchronization payload envelope capacity B in units of 7 VT groups (2 columns.times.7=14 columns). For example, if the VT sizes of all data accommodated in synchronization payload envelope capacity B are equal and 3, as shown in FIG. 6, the first and second units are accommodated in the second to the 29th columns (i.e., between the POH and the first unit of fixed stuff), the third and fourth units are accommodated in the 31st to 58th columns (i.e., between the first and second units of fixed stuff), and the fifth and sixth units are accommodated in the 60th to 87th columns (i.e., in the columns following the second unit of fixed stuff).
(4) When the VT size is 6, one column is allotted as one group, and the data is accommodated in synchronization payload envelope capacity B in the units of 7 VT groups (1 column.times.7=7 columns). For example, if the VT sizes of all data accommodated in synchronization payload envelope capacity B are equal and 6, as shown in FIG. 7, the units from the first to the fourth are accommodated in the second to the 29th columns (i.e., between the POH and the first unit of fixed stuff), the units from the fifth to the eighth unit are accommodated in the 31st to 58th columns (i.e., between the first and second units of fixed stuff), and the units from the ninth to the twelfth unit are accommodated in the 60th to 87th columns (i.e., in the columns succeeding the second unit of fixed stuff).
Since the SONET transmission signal is permitted to assume any one of the four VT sizes according to the format of the SONET transmission signal, a SONET transmission signal processing apparatus must necessarily specify the VT size of the SONET transmission signal. A method of specifying the VT size of the SONET transmission signal in the SONET transmission signal processing apparatus is briefly described below.
(1) If the VT sizes of all data accommodated in synchronization payload envelope capacity B are equal to 1.5, as shown in FIG. 4, the value "11," which is VT size information representing VT size=1.5, is placed into two predetermined bits of each first byte (indicated as "Byte 1" in FIG. 4) of the first unit, and the SONET transmission signal including the VT size information is transmitted to the SONET transmission signal processing apparatus.
(2) If the VT sizes of all data accommodated in synchronization payload envelope capacity B are processing apparatus detects the VT size from the data of the two predetermined bits of each first bytes of the first unit.
However, since a conventional SONET transmission signal processing apparatus operates in accordance with the VT size detected in accordance with the method described above from a SONET transmission signal transmitted to the apparatus, there is a problem that the apparatus may not operate, depending upon a SONET transmission signal received, with the VT size the user desires, and consequently, the reliability of the output signal of the SONET transmission signal processing apparatus is low. Further, when the data of the two predetermined bits representing the VT size is transmitted to the apparatus contains an error due to trouble in the transmission line or at the transmitter side, the SONET transmission signal processing apparatus operates in accordance with the wrong VT size. This factor also results in the decreased reliability of the SONET transmission signal processing apparatus.